The present invention relates to sacrificial coatings for ferrous metals, most particularly to hot dipped zinc base coatings containing substantial amounts of aluminum.
To produce hot dipped galvanized material, such as sheet or tubing, a metal article is immersed into a molten bath for a time sufficient to provide thereon a coating upon removal. Good coatings are characterized by a relatively superficial interaction, but good adherence, between the zinc metal of the molten bath and the metal article surface. Typically, zinc bath temperatures are held in the range of 440.degree.-465.degree. C. Higher temperatures increase bath fluidity and give better drainage of the excess hot dip coating material, thereby allowing thinner and more controlled coating thickness. But temperatures above 480.degree. C. are avoided because they tend to result in oxidation of the melt and harmful attack of the typical ferrous metal bath container. Other negative aspects are increased time for the coating to solidify and increased energy losses.
Lead and iron are normal impurities in commercial zinc. In addition, the zinc base hot dipped coatings of the prior art contain various alloying metals to improve the coating process or properties. Among the metals which have been controllably added are lead, tin, aluminum, cadmium, antimony, and magnesium. Tin, lead, and antimony are often used to affect spangle formation. Aluminum is a particularly desirable coating element, since it greatly enhances corrosion resistance. In addition small quantities of aluminum tend to lower melt temperature and inhibit the reaction between the coating material and the ferrous substrate or container. Of course, for amounts of aluminum over 10% the bath temperature is increased, and the foregoing advantages can be countered.
Roe et al. U.S. Pat. No. 3,320,040 states that aluminum controls the thickness of the intermetallic layer between the coating and the substrate. Above 3.5Al (all quantities herein refer to weight percent), it is indicated that the intermetallic layer was too thin, resulting in poor coating adherence. Leckie et al. U.S. Pat. No. 4,056,657 discloses the coating alloy Zn-5Al, together with an additive comprised of 0.1% of the material selected from the group of Pb, Sb, and Sn. For Zn-5Al alone, there is dross and the difficulty in getting a bright, smooth ripple free, properly spangled, surface. Lackie et al. states that lead reduces ripple but increases spangle size. But if aluminum is maintained within 5.+-.0.5 weight percent, 0.1% additive provides a formable coating having a good corrosion resistance.
High percentages of aluminum, from 3 to 17%, are disclosed in Lee et al. U.S. Pat. No. 3,505,043. In addition, the coatings contain 1.5 magnesium. Lee et al. U.S. Pat. No. 4,056,366 indicates how lead above 0.06 controls spangle formation in 0.2-17Al zinc base coatings. But, lead greater than 0.02 causes separation of the coating during deformation, after a coated article has been exposed to moist atmospheres; this is associated with intergranular attack of the coating. Improved coatings result when the lead content is maintained below 0.02 and antimony is present in amounts of about 0.15. Lee U.S. Pat. Nos. 4,039,478 and 4,128,676 mention the same problems and provide a different solution, comprising the inclusion of 0.1-0.15 of the magnesium. With this composition lead contents up to about 0.15 are tolerable. Borzillo U.S. Pat. No. 3,343,930 and Horton U.S. Pat. No. 3,952,120 generally disclose the value of silicon addition of 0.5-3% in zinc base coatings containing more than 25%Al. Silicon controls reactions at the coating and substrate interface in slow-cooling massive workpieces.
The prior art shows that compositions of zinc aluminum hot dip coatings can be relatively critical; there are certain known effects, but interactions between the diverse elements do not make optimum compositions easily deducible. The prior art of additives is largely concerned with adhesion and physical surface condition, while maintaining good corrosion resistance. Zinc aluminum sacrificial coatings provide protection by chemical action; in general, the greater the quantity or thickness present, the longer is the protective life. However, in some applications only very thin coatings are needed, and there is an economic incentive to controllably provide them.